sorry tuxduino, didn"t know HTH 
Of course you are right that during operation I should use Serial.available before each read. The problem here is just that there is nothing in the serial buffer, although I sent 1 2 ...97 241.
So my specific problem / question is why do I have a corrupt buffer values (jumps, which lead to a advanced end of the values stored in the buffer) indicated by the Serial.peek(). If that is solved I will include the if(Serial.available) {..=Serial.read;} then